Clear, clean waters form an essential linkage between koi and their keepers. Floating bead filters and UV lights provide the modern koi hobbyist with the opportunity to enjoy their fish without large burdensome filtration systems used by early koi enthusiasts. An excellent clarifier, the floating bead filter also provides biofiltration, while the UV light assures control of phytoplanktonic algae blooms in the sunniest ponds. Simplified backwashing approaches prevent filter clogging problems while constantly diverting captured solids away from the pond where they would contribute to depression of the essential oxygen supply . The result is a well aerated pond where fish can be clearly seen as they swim in waters that are free from toxic metabolites such as ammonia and nitrite.

Bead Filter History

Bead filters, utilizing floating plastic media, have been used since the mid 1970's as biofilters to support high density rearing of food and game fish in Idaho. Although successful, the operation of these early air washed filters was poorly understood and their use was limited. In the late 1980's, work at Louisiana State University demonstrated that a hydraulic washed bead filter was capable of supplying solids control and biofiltration for a high density catfish rearing system. These results stimulated additional academic interest in bead filters. Development of mechanically washed units (U.S. Patent Nos. 5,126,042 and 5,445,740) that were compact and simple to operate overcame many of the operational difficulties experienced by earlier designs. Shortly thereafter, the favored "bubble-washed" or "hourglass" configuration (U. S. Patent No. 5,126,042) was developed and tested for use on outdoor ponds. Since 1989, bead filters have been tested on systems holding food fish (such as tilapia, catfish, striped bass, trout), along with a wide variety of specialized applications (including ornamental fish, alligators, crayfish, crabs, oysters).

Koi, with their natural habit of "mouthing" bottom sediments, assure that no sludge deposits accumulate on the pond's bottom. The bead filters easily capture the sediments once they are disturbed by the koi, resulting in their rapid removal from the tank. In effect, the koi and bead filter work together to keep the pond clean. Both mechanically-washed and bubble-washed bead filters can be used to purify the water in outdoor pools. The filters are similar in operation, providing both clarification and biofiltration for the pond's waters. Large pools supporting several hundred pounds of koi are most commonly serviced using the mechanically-washed filters; whereas, smaller pools containing a moderate number of fish are well served by the bubble-washed units and their simple mode of operation. This document focuses on the use of the bubble- washed units since the use of the mechanically washed units is well documented elsewhere.

Description

Figure 1 illustrates the cross-section of the typical "bubble-washed" bead filter. When viewed from the side, the unit displays an hourglass shape and is referred to as an "hourglass" bead filter. The filter itself contains no moving parts. The filter casing has three major zones: I) the filtration chamber, 2) the washing throat, and 3) the expansion chamber. Water enters the filter through the slotted inlet pipe at the bottom of the filter, passes upward through the bead bed that is contained in the filtration chamber and exits through the slotted discharge pipe at the top of the filter. The inlet pipe also serves as the sludge discharge line. The filter is also equipped with an air inlet line that terminates below the center of the washing throat. The filter is normally operated with four valves (inlet, sludge, air, and discharge). However, check valves are sometimes used on the air inlet and water outlet lines to simplify the backwash operation.

Figure 1.
The "hourglass" or "bubble-washed" bead filters use a bed of floating plastic beads,
providing for simultaneous clarification and biofiltration.

The beads used in the filters are spherical in shape, approximately 1/8 inch in diameter, and are made from a food-grade, low density polyethylene plastic with a specific gravity of 0.91. They float and fill the filtration chamber when the filter is operational. The beads contain about 400 square feet of surface area for every cubic foot of beads that is used for bacterial growth. The beads are very durable and never have to be replaced.

Theory of Operation

Bead filters function both as a solids capture device and a biofilter. They operate very much like a submerged rock bed or undergravel filter except that they are designed to facilitate cleaning. Bead filters dramatically simplify operations by providing two of the most important water reconditioning processes, clarification and biofiltration in a single unit. Clarification is the process of removing suspended solids from water. Suspended solids in a pond consist of small particles of partially digested food, debris, algae, bacteria, clays, and silts, which are small enough to stay suspended in the water column for an extended period of time. Fine suspended solids tend to make the water opaque, interfering with the clarity of the water. The larger organic particles, on the other hand, represent a serious wasteload problem and can consume tremendous amounts of oxygen while adversely impacting the hygienic pond ecology. Bead filters are considered excellent clarification units, capable of maintaining display quality water at high waste loading rates. Bead filters remove suspended solids by four different mechanisms as the recirculated pond water is passed through the plastic bead bed. Physical straining is probably the most dominant mechanism for the larger particles (>80 microns). The bulk of the suspended particles (20-80 microns) are probably removed by interception, a subtle process caused by collisions between the particle and the bead media surface. The finest particles (<20 microns) are removed by bead filtration, but at a slower rate. It is believed that bioabsorption, the capture of particles by the bacterial biofilm, is the dominant process attacking these fine particles.

Biofiltration depends on the development of a filter bed that encourages the growth of beneficial bacteria, which extract dissolved chemicals from the water and convert them to particulate biomass or harmless dissolved compounds. Given the proper encouragement, the bacteria grow in a thin film, coating the surface of each of the 600,000 beads contained in every cubic foot of media. There are literally hundreds of species of bacteria at work in a biofilter. Most of the bacteria fall into the category of "heterotrophic" bacteria, which actively break down organic materials into carbon dioxide and water. The most critical however, are broadly described as nitrifying bacteria, consisting primarily of the genera Nitrosomonas and Nitrobacter. Nitrifying bacteria are responsible for the conversion of the toxic nitrogen forms of ammonia and nitrite to the relatively harmless nitrogen form of nitrate . Success and a wide variety of parameters that influence bead biofiltration have been identified. The pond hobbyist should be aware of these factors. However, the pond itself generally contains a very active population of nitrifying bacteria, so optimization of the filter's operation can be relaxed.

Sizing a Bead Filter

Table 1 presents the relationship between the volume of a bead filter, feedrate, pounds of fish, and pond size for typical applications. The bead filter sizes indicated are the minimum recommended. Larger bead filters will not necessarily perform better, but will require less frequent backwashing and have a larger safety factor.

Backwashing

Bead filters spend most of their time in the filtration mode (Figure 1). As the recirculating pond waters pass through the bed, solids consisting of fish feces, algae, zooplankton, debris, and biofloc accumulate, gradually clogging the filter. For most pond applications, the clogging process generally occurs over a one to two week period.

Figure 2

Figure 3Figure 4

Backwash Frequency

The bubble-washed bead filters are generally washed once or twice a week during the summer months in a typical koi pond application. They may be washed more often without adversely impacting nitrification performance. If the filters are not washed, they slowly clog, gradually shutting off the return flow to the pond. This decline in the return flow is usually visually evident, providing a convenient reminder for the need to backwash.

Bubble-washed bead filters perform better with frequent backwashing. Backwash frequencies of two to four times daily are used to carry heavy commercial loadings. Backwashing the filter once or twice a week is all that is required for most koi ponds. In the cooler winter months, backwashing frequency can be reduced to as little as once every two weeks.

Disposal of Backwash Waters

Bubble-washed filters are designed to lose 10-15 gallons of water per cubic foot of beads. The discharged waters will display the same high quality as the pond water; however, it will be burdened with a suspended solids load of 500-1500 mg/l, reflecting the scouring of the solids accumulated in the filter. These solids are generally already well decomposed by the bacteria in the bead filter and are generally discharged directly onto the lawn or a nearby garden. The backwash waters are usually enriched with phosphorus and nitrogen (principally nitrates), thereby, acting as a low grade fertilizer for the area of application. The solids show little potential for odor generation and are quickly assimilated into the underlying soils.

Alternatively, the backwash waters can be discharged directly to the household sewer line. The wash waters are entirely compatible with centralized or household treatment systems, which are based in a large part on the same bacterial communities that effectuate treatment in the bead filters.

Algae Control

Bead filters can not eliminate algae blooms (green water) in ponds exposed to strong sunlight. They are effective at removing suspended particles as small as 10 microns, but cannot harvest the fast growing small 5- 10 micron algae well enough to assure really clear water since they continually grow. To overcome this problem, the bead filters are often operated with an ultraviolet light (W). UV units produce a high intensity of light in the ultraviolet wavelengths which are very damaging to most living organisms. The bulbs are capable of killing most bacteria and algae on their first pass through the unit. The sizing of a UV light is critical to its success at combating planktonic algae. The pond's volume must be turned over more rapidly than the algae can grow (i.e., pond turnover rate must be greater than algae growth rate). During the hot summer months, many algae that may inhabit a koi pond can divide every six to twelve hours. Subsequently, the UV unit must be sized to process the entire pond's volume two to four times a day with an intensity of at least 15,000 µwsec/cm².

Oxygen

Bead filters with their powerful biofiltration and clarification capacities tend to keep the water in a koi pond very clean allowing pond keepers to substantially increase the koi population. As their numbers rise, the fish and bacteria will increase their demand for oxygen. At some point, additional aeration capacity will be dictated. Fish in heavily loaded ponds should be watched for respiratory stress, particularly in the early morning when dissolved oxygen levels are typically ebbing. Respiratory stress is indicated by a lack of appetite or when a large number of fish are seen at the surface of the water gasping. This problem will be most acute during the warmest days of summer went the feeding rate is high and oxygen saturation levels in the water are lowest. The problem can be remedied by addition of a supplemental waterfall, spray head or aerator to the system. Two aeration systems are definitely better than one, preferably plugged into lines served by separate circuit breakers.

A more subtle danger arises whenever the recirculation flow is interrupted. Suddenly, the entire fish population must depend on the natural ability of the pond to supply oxygen. Waterfalls, spray heads and air stones deliver oxygen 10-100 times faster than the pond surface alone. Expensive fish or favorite pets should be protected by maintaining a low fish population. If the fish population is too high to be supported by the pond's natural aeration, a 12 volt aerator or a compressed oxygen tank should be installed for backup. A simple way to determine the need for a backup aerator is to turn all your pumps off and observe the fish. If the fish come to the surface within a couple of hours, immediately turn your pumps back on and consider a reduction in population or a backup system. No damage to your fish will occur if the pumps are restarted as soon as the fish are seen gasping.

Intake Screens

Intake screen design and placement is an important but often overlooked aspect of pond design. Intake screens provided with submerged pool pumps are generally inadequate to deal with the quantity and variety of debris that is associated with an outdoor koi pond. They can clog frequently, incapacitating the filter endangering the fish and reducing aeration, while creating a maintenance headache.

Bead filters operate best when protected by a pre-screens with a mesh size of about 1/8 of an inch. This mesh size is commonly used in swimming pool screens and in-line catch baskets. Use of an easily accessed inline catch basket on the intake side of the pond's main recirculation pump will eliminate most problems with the pump and the bead filter. But, for ponds in the vicinity of trees and shrubs, an in pond screen box or slotted intake pipe is required for care-free pond operation. The intake structure should be positioned 8-12 inches above the deepest part of the pond where leaves and debris can settle below the intake structure without clogging it. The settled leaves can then be removed at your convenience. Placement of the intake structure above the bottom will not adversely impact water quality of the pond since the fine solids which control water quality will be suspended by the koi where they can be removed by the filtration system. A properly designed intake structure should require servicing no more frequently than once a week. Off bottom placement not only reduces cleaning frequency, but also, provides a safety reservoir for the fish should the wrong valve be left on and the pond accidentally pumped dry (almost).

Acknowledgments

This work was funded, in part, by the Louisiana Sea Grant College Program, an element of the National Sea Grant College Program, under the direction of NOAA, U.S. Department of Commerce. Pond sizing information was reviewed by Mr. Burt Nichols of Water Gems, Inc in Marion Texas. Dr. Ron James, President of Aqua Ultraviolet in Temecula, California provided technical support for UV light sizing criterion.

Aquaculture publications

Ms. Elizabeth Coleman, Coordinator
Office of Sea Grant Development
103 Wetland Resources
Louisiana State University
Baton Rouge, Louisiana 70803
(504) 388-6449
FAX: (504) 388-6331

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